(Supported in part by NIH GMS 40198, to C. Rieder and NSF MCB9420772, to B. McEwen) During mitosis (cellular division) in animal cells, many chromosomes initially attach to microtubules (MTs) from a single spindle pole and move toward that pole. When the unattached kinetochore (K) on these chromosomes attaches to MTs from the other pole, the sister Ks begin near-unidirectional movement away from the closest pole towards the spindle equator. During these congression movements, the most recently attached K exhibits MT minus-end directed motion towards its pole (P motion), while the other K exhibits MT plus-end directed motion away from its pole (AP motion). Based on in vitro observations that phosphorylation and a greater number of associated MTs favor plus-end directed motion, Hyman and Mitchison (C.H.S.Q.B. (1991)56745) have postulated that the accumulation of MTs biases the K toward AP motion, and that unequal rates of MT accumulation by sister Ks contribute to con gression. Using same-Cellular correlative video-enhanced DIC LM/3D EM techniques we found that KMt number was unimportant for balance of forces or initiation of congression. We also found that acquiring a full complement of kMts did not insure release of the anaphase checkpoint and that kMt acquisition is a relatively slow and high stochastic process. McEwen, B.F., A. B. Heagle, G.O. Cassels, K.F. Buttle and C.L. Rieder. (1997) Kinetochore-Fiber maturation in PtK1 Cells and its implications for the mechanisms of chromosome congression and anaphase onset. J. Cell Biol., 137:1567-1580.